Using an air-conditioned single-plant chamber, shoots of two dwarf beán cultivars were exposed for 5 oř 6 h to O3 concentrations, ranging from 0.3 to 0.8 pmol mol**. Diuing exposure water vapour, CO2, and O3 exchange rates of shoots were monitored with the aim to quantify possible differences between cultivars in sensitivity of stomata and mesophyll to O3 uptake. Aíter exposure changes in the water holding capacity of the treated leaves were also estůnated, combining pressure bomb, 3-gauge, and leaf chamber techniques. Rates of change of stomatal (Vg) and mesophyll (Vg,) conductances increased linearily with increase in the absorbed O3 flux via the stomata (Qg), the direshold for stomatal response being lower than that for mesophyll response. Above the threshold values of Qg, sensitivity of V„ to Qg was lower than ďiat of Vg. The water holding capacity of leaves decreased with increase in Qg, although no distinct threshold Qg was found. Cultivar differences in O3 sensitivities of Vg were statístically distinguishable on the stomatal segment of CO2 uptake route ordy.